The present invention relates to telecommunications, and more particularly to techniques for detecting information transmitted by means of a shared channel.
High Speed Downlink Packet-data Access (HSDPA) is an evolution of Wideband Code Division Multiple Access (WCDMA) specified in the Release 5 version of the Third-generation Partnership Project (3GPP) WCDMA specification. HSDPA introduces higher bit rates (up to over 10 Mbits/s) by using higher order modulation (16-QAM), multicodes (up to 15 with spreading factor 16), and downlink channel feedback information. Downlink channel feedback information is information, sent to the base station, regarding the downlink channel quality. The base station (BS), which in 3GPP terminology is called “node B”, uses this information to optimize modulation and coding for optimized throughput. Furthermore, Hybrid ARQ is also introduced on the physical layer in order to reduce the round trip delay for erroneous received packets.
HSDPA works according to the following. A User Equipment (UE), operating in connected mode, continuously transmits Channel Quality Index (CQI) reports to the HSDPA serving node B by means of the uplink (UL) High Speed Dedicated Physical Control Channel (HS-DPCCH). The CQI informs the serving node B about the instantaneous downlink (DL) channel quality in order to enable the node B to optimize the downlink throughput. The CQI could, for example, be a function of Signal to Interference Ratio (SIR), where the particular function depends on higher layer parameters (e.g., available HS-power, and the like). When the UE is scheduled by the node B and data packets will be transmitted to the UE, the HS Shared Control Channel (HS-SCCH) is used to inform the UE about information that the UE will use in the upcoming communication, such as information about the data packets and transport format, retransmission number, and the like.
The UE demodulates information transmitted on the HS-SCCH. If the information is directed to that particular UE (recall that this is a shared channel, so the information could be directed to another UE), the UE will receive and detect data packets transmitted on a High-Speed Physical Downlink Shared Channel (HS-PDSCH). The UE then sends either an acknowledgement (ACK) or negative acknowledgement (NACK) to the serving node B by means of the HS-DPCCH. Depending on whether an NACK or ACK was received, the node B will either retransmit the same packet (that can be combined with the erroneous detected packet and hence a coding gain can be achieved) or transmit a new packet, if there are any.
As can be seen from the above description, it is very important that the UE be able to detect the information on the HS-SCCH because without this detection it cannot receive the data at all. If the UE does not detect the HS-SCCH, then no ACK/NACK will be transmitted (because the UE is unaware that it was the intended recipient of a data packet). Consequently, there will be a Discontinuous Transmission (DTX) in the uplink HS-DPCCH ACK/NACK field. The node B could detect the DTX, which in turn could cause the packet to be retransmitted. This will result in a loss in throughput, however. Furthermore, the DTX may not be detected at all because the UL ACK/NACK is a binary signal. Hence, ACK means amplitude value +1 and NACK means amplitude value −1. “Nothing at all” (DTX) corresponds to no transmitted ACK/NACK bit; that is, in the base station detector, the output should be zero. However, due to noise and other problems, zero may be erroneously detected as +1 or −1, making it difficult to discriminate DTX from ACK and NACK (i.e., the probability of erroneous DTX detection will be quite large). If a DTX is interpreted as an ACK, then a retransmission invoked by a higher layer will be needed, resulting in several packets having to be retransmitted. This will result in a significantly reduced throughput. Hence, good HS-SCCH detection performance is important both from a UE throughput point of view and also from a system capacity point of view.
One simple solution to this problem is to retune the HS-SCCH detector such that the probability of a missed detection will be reduced. However, this comes at the price of an increased false alarm probability (i.e., the UE will “think” that it has received an HS-SCCH transmission when it has in fact not). Such false alarms would heavily increase the current consumption in the UE, resulting in lower talk time, and other associated problems. Consequently, this solution is not desirable.
The ability to accurately detect transmissions on the HS-SCCH is related to the power level used to transmit information on the HS-SCCH. However, the exact power setting for HS-SCCH is up to the serving node B, and the strategy for setting this transmission power level could be different from one node to another. For example, HS-SCCH transmission power could be related to the power level used on the Dedicated Physical Control Channel (DPCCH) (i.e., power controlled). Alternatively, it could be related to the CQI (e.g., the current HS-PDSCH SIR). In yet another alternative, the transmission power on the HS-SCCH could be set to a constant output power (e.g., as the Common Pilot Channel, or “CPICH”). Other methods are also possible.
All of these different methods have different pros and cons. For instance, a power offset to DPCCH might cause the HS-SCCH to experience performance problems during Soft HandOver (SHO). This is because the HS-SCCH does not support SHO, while the DPCCH does, so that there is no SHO gain for the HS-SCCH. Hence, for highly asymmetric DL:s with a majority of the power on the non-HSDPA serving the DPCCH, very poor HS-SCCH detection performance will result.
On the other hand, relating the HS-SCCH power to the CQI means that the node B will have to guess about the relationship between the HS-PDSCH and HS-SCCH reception performance. But the node B may not have a good basis for making any such guesses. For example, the UE might use some kind of advanced detector (e.g., a Generalized-RAKE, or “G-RAKE” receiver) for receiving the HS-PDSCH, but, due to challenging real-time requirements, not use similar technology for receiving the HS-SCCH. Hence, the relative performance between the UE's HS-SCCH reception and the HS-PDSCH reception could be different compared to the “guesses” in node B. As a consequence, the UE might experience HS-SCCH reception performance that is poor for some CQI:s and better for others. This will again result in a loss of throughput.
Finally, consider the case in which the HS-SCCH transmission power is set to a constant level. Here, a high level could be chosen in order to ensure that the signal is capable of reaching the cell border. This is in some sense a good solution from the UE's point of view, since it enables the use of a simpler detector that uses less current while still providing good performance. However, this is not a desirable solution from the point of view of the serving node B because it results in capacity loss.
It is apparent from the above discussion that there is a need for methods and apparatuses in the UE that will optimize the detection of information transmitted on the HS-SCCH while still allowing the serving node B to employ any of a number of HS-SCCH power setting strategies. While this problem has been described with specific reference to HSDPA communication systems, it will be readily apparent that similar problems can exist in communications systems adhering to other standards. Thus, solutions are needed in these other systems as well.